Update colbert_utils.py

Setting use_faiss to True.

colbert_utils.py

@@ -39,6 +39,6 @@ def build_colbert_model(
         index_name=index_name,
         max_document_length=180, 
         split_documents=True,
-        use_faiss=False # cannot get it to work...
+        use_faiss=True # set to True if faiss working properly in current env
     )
     return model